Matrix element voltage sensing for precharge

Abstract

A method and apparatus to calibrate an LED matrix display such that a driver will provide a proper precharge voltage to LED elements within the display during a scan period. A current is driven through a calibration element, and a voltage reflecting the steady-state element voltage is measured and stored as a calibration value. A processor controls whether to precharge during the calibration cycle, and determines when the calibration cycle is completed. During subsequent normal scans, a driver applies a voltage based on the stored calibration value to rapidly precharge parasitic capacitance associated with a display element to a proper value, and also drives a selected current through the device.

What is claimed is:  
     
       1. A method of determining a precharge voltage for current-driven devices in a matrix, the method comprising: 
       driving a selected current through a target device;  
       determining a calibration time for a steady-state voltage developed by the target device conducting the selected current, the calibration time being based on a comparison between a plurality of display conduction voltage samples;  
       sampling a calibration display conduction voltage at the calibration time;  
       producing a calibration value representing the calibration voltage; and  
       storing the calibration value for later use during normal operation.  
     
     
       2. A method of calibrating a display device having at least one electroluminescent element and a display driver, the method comprising: 
       (i) predetermining a measurement period of time;  
       (ii) applying a first current to the element from a current start time;  
       (iii) continuing the current for the predetermined period of time;  
       (iv) measuring a display device voltage reflecting a voltage of the electroluminescent element at the end of the predetermined period; and  
       (v) storing a representation of the measured voltage in a memory as a calibration value for later retrieval during a non-calibration mode of the display device.  
     
     
       3. The method of  claim 2 , further comprising selecting the predetermined period of time to be sufficiently long to ensure that the electroluminescent device voltage reaches equilibrium within a single predetermined period of time. 
     
     
       4. The method of  claim 3  comprising selecting the predetermined time to be between about 0.1 millisecond and 10 milliseconds. 
     
     
       5. The method of  claim 2 , further comprising repeating steps (iii) and (iv) after successive predetermined periods of time within a single calibration cycle until successive measurements satisfy predetermined difference characteristics. 
     
     
       6. The method of  claim 5 , wherein the predetermined period of time is between 5 microseconds and 200 microseconds. 
     
     
       7. The method of  claim 5 , wherein step (v) is performed only after the successive measurements satisfy criteria indicating achievement of steady state. 
     
     
       8. The method of  claim 5 , wherein step (i) includes predetermining a plurality of predetermined periods having different durations. 
     
     
       9. The method of  claim 2 , wherein applying a first current to the element includes applying a current to at least one column of an OLED array. 
     
     
       10. The method of  claim 2 , further comprising retrieving the voltage measurement representation during a normal display mode, and precharging a plurality of display element columns at a beginning of a scan to a precharge voltage based upon the retrieved voltage measurement representation. 
     
     
       11. The method of  claim 2 , further comprising precharging a voltage on the display element prior to starting the first current. 
     
     
       12. The method of  claim 2 , further comprising executing instructions on a processor to determine the start time and the predetermined period. 
     
     
       13. A calibration circuit for a display device, comprising: 
       a current source configured to provide a known current to the display device beginning at a calibration current start time;  
       a measurement circuit configured to sample display device voltages at directed sample times and create representations of the sampled voltages;  
       a controller configured to  
       select a steady-state sample time corresponding to a steady-state response to the known current,  
       direct the measurement circuit to sample the display device voltage at the steady-state sample time to create a corresponding steady-state voltage representation,  
       coordinate transfer to memory of the steady-state voltage representation, and  
       direct retrieval of the steady-state voltage representation during a non-calibration mode of operation; and  
       a memory configured to store the steady-state voltage representation for retrieval during the non-calibration mode of operation.  
     
     
       14. The calibration circuit of  claim 13 , wherein the controller is configured to determine the start time, and to select the sample time sufficiently long after the start time to ensure that steady state has been reached by the sample time. 
     
     
       15. The calibration circuit of  claim 14 , wherein the unit is configured to wait for a period of time after a duration ranging from about 100 microseconds to about 10 milliseconds. 
     
     
       16. The calibration circuit of  claim 13 , wherein the unit comprises an analog-to-digital converter that is configured to convert the sampled voltage into a digital value for retrieval during the non-calibration mode. 
     
     
       17. The calibration circuit of  claim 13 , wherein the current source is configured to apply the current to at least one column driver associated with an OLED array. 
     
     
       18. The calibration circuit of  claim 13 , wherein the controller includes a comparator which compares values of successive column voltage samples. 
     
     
       19. The calibration circuit of  claim 18 , wherein the controller selects a sample time between 5 and 200 microseconds between successive samples. 
     
     
       20. The calibration circuit of  claim 19 , wherein the steady-state voltage representation is determined when values of successive samples meet range criteria as determined by the controller. 
     
     
       21. The calibration circuit of  claim 13 , further comprising a precharge source, wherein the controller includes a precharge value generator which controls the precharge source output during a normal display mode based upon the stored steady-state voltage. 
     
     
       22. The calibration circuit of  claim 13 , further comprising a precharge source, wherein the controller includes a precharge value generator which directs the precharge source to apply a precharge voltage to the display device during a precharge period. 
     
     
       23. The calibration circuit of  claim 22 , wherein the start time is preceded by the precharge time. 
     
     
       24. The calibration circuit of  claim 23 , further comprising a means for precharging a display device element based upon the stored steady-state voltage representation. 
     
     
       25. A calibration circuit for a display device, comprising: 
       a means for providing a known current to the display device beginning at a calibration current start time;  
       a means for measuring display device sampled voltages at directed sample times to create representations of the sampled voltages;  
       a controller configured to  
       select a steady-state sample time corresponding to a steady-state response to the known current,  
       direct the measurement circuit to sample the display device voltage at the steady-state sample time to create a corresponding steady-state voltage representation,  
       coordinate transfer to memory of the steady-state voltage representation, and  
       direct retrieval of the steady-state voltage representation during a non-calibration mode of operation; and  
       a memory configured to store the steady-state voltage representation for retrieval during the non-calibration mode of operation.